1. Field of the Invention
The present invention relates to a photodiode.
2. Description of the Related Art
Recently, in order to cope with a dramatic increase in the capacity of networks, epitomized by the widespread use of the Internet, development of high-speed optical transmission systems has been sought. Today, optical fibers having comparatively large apertures that facilitate alignment, such as plastic optical fibers, multimode glass optical fibers, and the like, are drawing increased attention as optical fibers for use not with trunk line systems but “last one mile” transmission systems, including subscriber line systems.
Further, it is demanded that a photodiode (PD), transimpedance amplifier (TIA), semiconductor laser (LD), and the like are monolithically formed on a Si substrate using an ordinary CMOS process that allows mass production at low cost for applications, in particular, to the “last one mile” transmission system.
In the mean time, in order to produce a photodiode capable of responding to light having a wavelength longer than that of visible light with high speed and high sensitivity, a method for manufacturing a photodiode by epitaxially growing a Ge thin film on a Si substrate is proposed as described, for example, in U.S. Pat. No. 6,635,110 (Patent Document 1), and “High-Speed Germanium-on-SOI Lateral PIN Photodiodes”, G. Dehlinger et al., IEEE Photon. Technol. Lett., Vol. 16, Issue 11, pp. 2547-2549, 2004, (Non-Patent Document 1). Note that, in principle, GaAs, InGaAs, and InGaAsP may also be used other than Ge, but the use of such materials poses technical difficulties and not realistic. On the other hand, Ge has already started to be used in a manufacturing process of high-speed TIA and the like, as SiGeBi-CMOS process, and would be a preferable material also from the viewpoint of contamination.
Note that if a substrate on which a photodiode is formed has many crystal defects, problems of decreased sensitivity, increased dark current, and the like may result. Generally, it is difficult to obtain quality crystals with less crystal defects such as dislocation and the like by epitaxially growing a different material. However, it is reported in the Patent Document 1 that a quality Ge film may be grown on a Si substrate. More specifically, it is reported that high quality Ge thin film may be grown by performing selective epitaxial growth on a region smaller than or equal to 40 μm×40 μm, preferably smaller than or equal to 20 μm×20 μm. The Non-Patent Document 1 describes that it is possible to create a Ge lateral PIN photodiode of 10 μm×10 μm or 30 μm×30 μm on a SOI substrate using this technique.
Further, MSM (metal-semiconductor-metal) photodiodes, lateral PIN photodiodes, and the like, in which electrodes are arranged laterally, i.e., in the direction parallel to the surface of the substrate in a finger pattern, are known as optical receivers suitable for high-speed and large aperture optical fibers. U.S. Pat. No. 5,777,390 (Patent Document 2) proposes to use transparent electrodes for either one of the types of electrodes in such types of photodiodes in order to alleviate a trade-off between the sensitivity and high-speed response.
Generally, in a photodiode having a transparent electrode, the sensitivity and high-speed response become a trade-off due to (1) a large time constant arising from a high resistance, and (2) generation of low mobility holes even under the positive electrode if the active layer is an intrinsic or lightly doped N type layer, and the travel distance of the holes becomes long. The structure described in the Patent Document 2 tries to reduce the travel distance of the holes by using a transparent electrode (different in material and clearness from that of the other type of electrode) as the electrode for collecting low mobility holes, i.e., negative electrode, while preventing holes from being generated under the positive electrode by using a low resistance nontransparent electrode as the positive electrode, in order to alleviate the trade-off.
However, when manufacturing a photodiode having a large aperture (approximately, a radius of 50 μm, and a cross-sectional area of not less than 7500 μm2) that facilitates optical fiber alignment by simply using the techniques described in Patent Document 1 or Non-Patent Document 1, the problems of decreased sensitivity and increased dark current are recognizable.
Further, in the structure described in Patent Document 2, the use of long transparent electrodes results in high resistance, and, in effect, there still exists the problem of the trade-off between increased sensitivity and speed.